A high power factor and low lattice thermal conductivity are two essential ingredients of highly efficient thermoelectric materials. Although monolayers of transition-metal dichalcogenides possess high power factors, high lattice thermal conductivities significantly impede their practical applications. Our first-principles calculations show that these two ingredients are well fulfilled in the recently synthesized Pd 2 Se 3 monolayer, whose crystal structure is composed of [Se 2 ] 2– dimers, Se 2– anions, and Pd 2+ cations coordinated in a square-planar manner. Our detailed analysis of third-order interatomic force constants reveals that the anharmonicity and soft phonon modes associated with covalently bonded [Se 2 ] 2– dimers lead to ultralow lattice thermal conductivities in Pd 2 Se 3 monolayers (1.5 and 2.9 W m –1 K –1 along the a - and b -axes at 300 K, respectively), which are comparable to those of high-performance bulk thermoelectric materials such as PbTe. Moreover, the “pudding-mold” type band structure, caused by Pd 2+ (d 8 ) cations coordinated in a square-planar crystal field, leads to high power factors in Pd 2 Se 3 monolayers. Consequently, both electron- and hole-doped thermoelectric materials with a considerably high zT can be achieved at moderate carrier concentrations, suggesting that Pd 2 Se 3 is a promising two-dimensional thermoelectric material. Our results suggest that hierarchical chemical bonds, that is, coexistence of different types of chemical bonds, combined with a square-planar crystal field is a promising route for designing high-efficiency thermoelectric materials.
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Naghavi et al. (2018) studied this question.